The invention relates in general to telecommunication systems and circuits therefore, and more particularly to a supply voltage feeding circuit that is configured to provide a supply voltage for a POTS (Plain Old Telephone Service) subscriber line pair.
In POTS systems, an analog telephone is coupled via a subscriber line pair to a telephone exchange. As the analog telephone does not have its own voltage supply, it is operated by a direct current received from the telephone exchange via the subscriber line pair. To this end, the telephone exchange comprises a supply voltage feeding circuit for generating a direct voltage and applying it to the subscriber line pair. By off-hooking the analog telephone, a contact closes and the supply voltage causes the supply current to flow through the subscriber line pair and to power the analog telephone.
At the telephone exchange, the supply of DC voltage on the subscriber line pair has to be regulated in order to maintain a certain current to feed or bias the analog telephone. Therefore, the DC current on the subscriber line pair is detected and the supply voltage feeding circuit for controlling the DC voltage applied to the subscriber line pair comprises a feedback loop which regulates the DC voltage in response to the detected current flowing through the subscriber line pair. In other implementations, this feedback loop may be formed for detecting the line voltage and feeding a line current. Voltage regulation is known as “battery feeding” and is performed continuously during the whole period of the call. Therefore, line feeding noise generated by the supply voltage feeding circuit has to be minimized. Typically, this is done by a low pass filter blocking AC signal contributions from entering the subscriber line pair.
Besides battery feeding, the supply voltage feeding circuit in the telephone exchange has to provide the ring functionality. During ringing, a low frequency AC voltage in the range of 15 Hz to 60 Hz is fed to the subscriber line pair. However, the low pass filter used for filtering off the line feeding noise may undesirably impair or even block the AC ring signal. As a result, it may be necessary to use a low pass filter with variable cut-off frequency or otherwise to increase the complexity of the circuit in order to guarantee for low noise battery feeding as well as effective ringing.